1. Field of the Invention
The present invention relates to diffusion bonding of space age metals, such as titanium and zirconium and their alloys, and particularly to low pressure joining techniques which utilize a eutectic fugitive liquid phase. More specifically, this invention is directed to diffusion brazing and to foil type braze alloy preforms for use therein. Accordingly, the general objects of the present invention are to provide novel and improved methods and materials of such character.
2. Description of the Prior Art
There has been a long standing desire, particularly in the aerospace and nuclear industries, for production techniques applicable to the joining of components comprised of materials such as zirconium, titanium and their alloys. For purposes of explanation, the discussion below will be directed to the joining of members formed from titanium alloys such as Ti-6Al-4V (nominally 6 percent aluminum, 4 percent vanadium, balance by weight titanium).
It is well known that, in working with alpha or alpha-beta titanium alloys, temperature must generally be limited to a range below the beta transus. Titanium has two lattice forms; i.e., alpha or hexagonal close packed and beta or body center cubic. In alpha-beta alloys excess amounts of the beta form or transformed beta Ti, which are characterized by embrittlement, will occur if the alloy is subjected to a temperature in excess of the beta transus which, for Ti-6Al-4V, lies in the range of 1790.degree. F to 1850.degree. F; the melting point of Ti-6Al-4V being approximately 3000.degree. F. Thus, titanium and titanium alloys can not be bounded to each other by conventional fusion welding techniques without formation of the beta phase in the weld zone and in part of the heat effected zone. In view of the inability to weld titanium alloys without detrimental property effects, various diffusion bonding processes have been proposed, and in some cases utilized, in the prior art.
The prior art diffusion or braze bonding techniques applicable to titanium and titanium alloys may be generally classified by the form of an intermediate material; i.e., either solid or powder; positioned between the members to be joined. In both cases the intermediate forms of eutectic liquid and bonding is achieved by virtue of an exchange of atoms across the interface between the parts; the liquid metal enhancing the exchange and overcoming mismatch between the facing surfaces at the desired joint. Use of an intermediate which forms a eutectic alloy with the titanium alloy is, of course, highly desirable because it lowers the maximum temperature to which the titanium need be subjected during the bonding process. Thus, to summarize, in prior art diffusion bonding or brazing of titanium and/or titanium alloys an intermediate which forms a eutectic alloy having the lowest possible melting point is selected in the interest of limiting the detrimental property effects associated with the formation and growth of beta phase titanium. Additionally, the intermediate must form, with the titanium and or titanium alloys, ductile joints which are characterized by an absence of continuous interfaces of brittle compounds. Thirdly, the intermediate must be susceptible to storage without deterioration or contamination. In use the intermediate must be capable of placement without undue difficulty, as a thin layer, between the surfaces to be joined. Also, the intermediate should permit the formation of joints which are of consistent quality and are thus reproducible. The intermediate must, of course, be soluble in the base material and the amount of solid state diffusion necessary to form a satisfactory joint should be minimized.
Prior art braze bonding tecniques employing solid state preforms have failed to meet one or more of the above-discussed criteria. Thus, for example, brazing processes which have employed silver based alloys as the intermediate have produced joints with poor stress-corrosion resistance.
Prior art brazing techniques utilizing an intermediate in powder form have been characterized by high expense and a severe reproducibility problem. The high expense may be attributed to both the cost of the powder and the difficult manipulative steps associated with spraying or otherwise distributing the powder on the parts to be joined. The lack of reproducibility is most often attributed to contamination, particularly oxidation of the powder, which raises the brazing temperature. Particularly troublesome is the fact that the only way the effect of powder contamination can be detected is by checking bonded joints.
It is especially noteworthy that many prior art intermediates, and particularly the rather well known Ti-Cu-Ni alloys, have required temperatures in the range of 1750.degree. F to 1800.degree. F to achieve a bond. These temperatures are sufficiently high to initiate the formation of the beta phase in Ti-6Al-4V. In an effort to overcome the embrittlement problem inherent in operating in this temperature range resort has often been had to titanium alloys such as Ti-8Al-1Mo-1V which has a higher beta transus than Ti-6Al-4V. However, as is well known, Ti-6Al-4V is less expensive and much easier to shape into the desired part than are other titanium alloys having comparable mechanical and physical property characteristics.
It is also noteworthy that most prior art diffusion bonding techniques, often in an effort to reduce the requisite temperature, have called for the application of considerable pressure across the desired joint. The necessity of applying high pressure is a substantial deterent to utilization of these techniques in production in view of the size and expense of the presses required.